This invention relates to a process for solvent refining of petroleum. Numerous solvents have been used to separate aromatic and paraffinic constituents in hydrocarbon streams. Such separation is desirable because the aromatic constituents in gasoline fractions contribute toward high octane numbers, while the straight chain paraffinic constituents greatly reduce octane numbers. Furthermore, pure benezene, toluene and xylenes are important raw materials in the petrochemical industry. Paraffinic hydrocarbon streams are used as constituents of heating oils and jet fuel.
The solvents used in such extraction processes must meet critical requirements of (1) being only partially miscible with the hydrocarbon feed stream (2) having a high hydrocarbon extraction capacity and (3) having a high selectivity for aromatic hydrocarbons. High capacity is important since this reduces the amount of solvent necessary to achieve the desired separation and thereby reduces the size of the vessels and associated pumps needed for recirculation. Selectivity (.beta.), is also important. This is defined as ##EQU1## Solvents having a high selectivity require fewer stages of extraction to achieve a given degree of separation. This, in effect, reduces the height of the extraction vessel. Generally speaking, the higher the capacity of the solvent the lower the selectivity. Therefrom some compromise is generally made between these properties in selecting a suitable solvent for the separation desired.
Other desirable characteristics of a good extraction solvent include the following: (4) low molecular weight; (5) non-corrosiveness to the equipment at operating conditions; (6) complete stability under operating conditions; (7) easy recoverability from the hydrocarbon products; and (8) low cost.
No extraction solvent has yet been discovered which satisfies all of the foregoing requirements for the separation of paraffinics and aromatics in hydrocarbon streams. A number of solvents have been used including sulfolane, N-formyl morpholine, various glycols and mixtures thereof, etc. The use of most of these solvents requires, subsequent to the extraction step, that either the solvent or the hydrocarbon be removed from the aromatic-rich solvent phase by distillation. This requires a large amount of heat which is a major cost item in the extraction process.
There is great incentive in seeking an extraction solvent which would be lower in cost than those heretofore used commercially. Furthermore, there is additional incentive in seeking a solvent which can be separated from the aromatic-solvent phase without the necessity of distillation. This invention provides a solvent meeting these requirements. The use of said solvent results in a process for separation of aromatic and paraffinic hydrocarbons which is economically attractive in that the solvent is quite inexpensive and can be separated from the aromatic-rich phase by simple lowering of temperature, rather than by distillation.